This invention relates to a magnetic-based transducer for measuring a displacement, particularly a linear displacement or position measurement. The invention still further relates to a transducer assembly, in particular a magnetic-based transducer and transducer assembly.
Magnetic transducer technology has gained wide acceptance for measuring torque in shafts or other parts for transmitting torque. Magnetic-based torque transducers have found application in non-contacting torque sensors particularly for a shaft which rotates about its longitudinal axis. One or more magnetic regions, used as one or more transducer elements, are created in or on the shaft to emanate a torque-dependent magnetic field component external to the shaft which is detected by a sensor arrangement that is not in contact with the shaft.
The principles used in measuring torque or force may also be adapted to measuring displacement. The following description will be given in the context of shafts but will be understood to apply to measuring displacement in parts in general, unless the context otherwise requires.
One class of magnetic region used as a transducer element in magnetic transducers is self-excited in that it is a region of permanent or stored magnetisation which emanates an external field. The transducer region is sometimes referred to as xe2x80x9cencodedxe2x80x9d in that a predetermined configuration of magnetisation is stored in it.
A transducer element may be created in a region of stored or permanent (remanent) magnetisation in a ferromagnetic integral region of the shaft or part. Transducer elements of this kind are disclosed in published PCT applications WO99/56099, WO01/13081 and WO01/79801. The stored magnetisation may be of the kind known as circumferential in an integral region of a ferromagnetic shaft as disclosed in WO99/56099 or it may be a circumferentially-magnetised ring secured to the shaft as disclosed in U.S. Pat. No. 5,351,555. Another form of stored magnetisation is an integral portion of a shaft in which the stored magnetisation is in an annulus about the axis of the shaft and is directed longitudinally, that is in the direction of the shaft axis. One kind of longitudinal magnetisation is known as circumferential (tangential)-sensing longitudinal magnetisation and is disclosed in WO01/13081; another kind is known as profile-shift longitudinal magnetisation as disclosed in WO01/79801.
All these forms of transducer magnetisation are xe2x80x9cD.C.xe2x80x9d magnetisations in the sense they operate with unipolar magnetic fluxes generated by the remanent magnetism stored in permanent magnets.
The sensor devices used with self-excited transducer elements may be of the Hall effect, magnetoresistive or saturating core type. These sensor-devices are sensitive to orientation. They have an axis of maximum response, and an orthogonal axis of minimum response.
Another class of magnetic transducer region is externally excited by an energised coil wound about the region. One form of externally-excited transducer is the transformer type in which the region couples an excitation winding to a detector winding. For example the permeability of the transducer element may be torque dependent. The transformer-type of transducer is A.C. energised. An example of a transformer-type of transducer is disclosed in EP-A-0321662 in which the transducer regions are specially prepared to have a desired magnetic anisotrophy at the surface.
The measurement may need to be done in circumstances where there are interference magnetic fields of a largely unknown and unpredictable type. Such fields are likely to arise in production plants and such like environments. Magnetic field interference can arise from unipolar fields, noise spikes and xe2x80x9cA.C.xe2x80x9d fields at the local powerline frequency, among others.
Another form of externally-excited transducer element is disclosed in WO01/27584 in which the magnetisation of a region of a shaft arises from energisation of one or more coils are mounted coaxially with a shaft in which an applied torque is to be measured. The coils are axially spaced and define a transducer region therebetween. The coils are connected in series to generate magnetic fields in the same direction in the transducer region between the coils. The coils are energised to induce a longitudinal magnetic field of a given polarity. The longitudinal field in the transducer region is deflected in direction and to an extent dependent on torque applied to the shaft to produce an external circumferential (tangential) magnetic field component that is a function of torque. The axially-directed component of the field is separately detected to provide a reference against which the circumferential component is measured. If the coils are D.C. energised the resultant magnetic field may be interfered with by other local fields, e.g. fields extending along the shaft whose torque is to be measured.
In the torque measurement system disclosed in WO01/27584, the pair of spaced coils is A.C. energised at a frequency selected to be distinguishable from noise frequencies, e.g. mains power frequency, and the sensor output is also detected in a frequency-selective manner. The torque-dependent xe2x80x9cA.C.xe2x80x9d or alternating polarity (AP) magnetic field component is detected. By using A.C energisation interfering D.C. fields can be discriminated against and the A.C. frequency can be selected to discriminate against interference as the local power line or supply frequency, e.g. 50 or 60 Hz. The detection may be synchronous with the A.C. energisation. The external field to be sensed is enhanced by a pair of spaced collars of magnetic material attached to the transducer region to aid the establishing in a recess between the collars of an external component of the longitudinal field in the transducer region. A sensor arrangement responsive to a torque-dependent magnetic field in the circumferential (tangential) arrangement is disposed in the recess.
The just-described transducer has the advantage that the transducer region does not have to be encoded with a stored magnetisation. Nonetheless a transducer region has to be defined between a pair of spaced coils.
The prior art also discloses various magnetic field sensor arrangements using two or more sensor devices to compensate for potential measurement errors and to assist in nullifying the effects of interfering fields. Although small, such sensor arrangements may be difficult to accommodate in situations where space is at a premium.
The invention has been developed in relation to measuring the linear movement of a clutch mounted on a gear shaft housing. A particular problem that arises is the requirement that the main elements of the transducer are to be mounted within the housing which is of a ferromagnetic material. However, the measurement technique described herein is of more general utility and is not limited to the application to clutches. The transducer system to be described provides a non-contacting measurement of the displacement of the clutch mechanism through a ferromagnetic wall provided by the housing of the gear shaft.
Aspects and features of the present invention for which protection is contemplated are set out in the claims following this description.
The present invention seeks to provide a technique which affords a magnetic-based transducer greater immunity from interfering fields. It also can be implemented in a compact form using a single sensor device for detecting a displacement-dependent magnetic field component. To this end a transducer element has a displacement-dependent field created therein by energisation of a pair of spaced coils or a single coil. The transducer region is subjected to short duration magnetic pulses of alternating polarity and a differential measurement is made on these pulses as detected by a sensor arrangement. Specifically the differential measurement is a peak-to-peak measurement Such a measurement can be satisfactorily realised with the aid of a single sensor device acting at a single location.
As will be explained hereinafter, the present invention can be implemented in hostile environments such as those subject to shocks and where interference fields of an unpredictable nature may occur.
Aspects and features of this invention are set forth in the claims following this description.